High speed veneering



Dec. 8, 1964 A. ELMENDORF 3,150,543

HIGH SPEED VENEERING Filed June '7, 1961 ,f2/P555 ampi/WMP@ 525?- INV EN TOR. BY//w/ff 7W/rn, Pdf/@r KfW/ar United States Patent O 3,160,543 HIGH SPEED VENEERNG Armin Elmendorf, Seti Charleston Road, Palo Alto, Calif. Filed .lune 7, 1961, Ser. No. 115,384 4 Claims. (El. iSd-396) This invention relates toa high-speed veneering process and in particular to a veneering process using tmn veneer.

A primary purpose of the present invention is a veneering process which provides substantial economies in both material and labor.

Another purpose is a high-speed veneering process which substantially reduces the pressing time in the application of veneer to a substrate.

Another purpose is a veneering process in which onlyV one surface of the substrate is veneered.

Another purpose is a high-speed Veneering process of the type described in which a given log can produce from 4 lto 6 times the veneered panel footage obtainable heretofore. i

Other purposes will appear in the ensuing specification, drawing and claims.

Ihe invention is illustrated diagrammatically in the attached ligure which is al graph of pressing time vs. temperature.

TheA present high-speed veneering process is the result yof extensive experimentation in .the manufacture of ply- Wood. This invention has specialreference to the veneering :of ligno-cellulosic boards with hardwood veneer. it is referred to as a high-speed veneering process in view of the fact that the pressing time has been reduced to about 5 to l0 seconds as compared with conventional pressing times of from 2 to 4 minutes.

Various economies are possible with the present process. First -of all, l have found that it is possible to apply veneer to only one side of a substrate and yet obtain a llat panel. Heretofore, conventional veneering dictated that both sides of every panel be veneered to provide a llat panel. A further economy results in the reduction of the veneer thickness. It is now possible to cover two to three tirnes the area Wir-.h a given log that was possible using conventional procedures and conventional veneer thicknesses. in my process, I use veneer from 1/8 inch to 1/85 inch in thickness, Whereas conventional face veneers have a thickness of between iVm inch and 1/28 inch. Combining the reduction in thickness With the elimination of veneer on one side of the panel results in increasing the panel footage of a given log by 4 to 6 times.

la conventional veneering procedures, a substrate, such as core veneer, particle board or hardboard, has adhesive applied to both faces :of the substrate. The face veneers are bonded to the substrate Whilevthe adhesive is fresh and contains moisture.v Many attempts have been made to lbond veneer lto a substrate with a dry adhesive, such as powder casein glue7 in combination with green veneer. Such attempts Vhave never met with commercial success, primarily because the bond has not been satisfactory. Satisfactory bonds may be obtained with dry adhesive in the form of a thin iilm of paper impregnated with a synthetic resin. Such adhesives usually require at least three minutes for curing, hence there is no pressing economy whenV compared with conventional procedures using wet adhesives which normally require about 2 to 4 minutes for curing and pressing.

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Many attempts have also been made to bond veneer with momentary pressure, for example by means of hot rolls. Such procedures have not been commercially satisfactory, due primarily to the fact that veneers lose hygroscopic moisture quickly when heated with a hot roll. The veneers will shrink which results in a poor bond.

l have found that hot-plate pressure results in a blond superior to that obtained with roller pressure, even when the roller pressure is applied repeatedly at short intervals. In the application `of hot-plate pressure, there is, however, a lower limit on the period Iof time required to activate the adhesive adequately for good bonding. If very thin veneers are used and the adhesive is dry, at least 3 seconds must be allowed to activate the adhesive adequately `for good bonding, using conventional pressing temperatures and pressures. Whenl l refer to very thin veneers, i mean veneer having a thickness in the range of 14S to lg, inch. if veneers of conventional thickness are used, much longer pressing times are required to get satisfactory bonding. Conventional veneers must be maintained under hotplate pressure for at least one minute and preferably from 2 to 4 minutes to provide a satisfactory bond. If veneers of conventional thickness are used and the pressing time is less than 6G seconds, the bond is unsatisfactory. Y The bond is quite unsatisfactory for all pressing times below l2 seconds. In the present process, a very satisfactory bond is obtained Withpressing times of l2 seconds or less, but in any event the pressure must be maintained Vforet least 3 seconds.

l have found that theveneers used in my process must contain at least part of the equilibrium hyduoscopic moisture. rEhe bond obtained ifrtheveneers are oven dried and the pressing is done With a dry adhesive is not satisfactory. The moisture in the veneers will be used to activate the dry adhesive.

in my process, may use conventional adhesives of the type requiring Water in'their preparation and which dry by the evaporation of their water content. Conventional urea formaldehyde adhesives, for example, are satisfactory, as are many other adhesives normally used in veneering processes. The first step lin my ethod is to apply the adhesive, preferably in an aqueous solution, and in an amount considerably less than that used in conventional plywood manufacture, to the surface to be veneered. About 5 to l0 lbs. of dry adhesive per thousand square feet of surface is a satisfactory miount. After the adhesive is spread, it may be dried by any conventional drying procedure which does not cure the adhesive during drying. Air drying, s in a roller dryer, is satisfactory. lt is preferred to apply the veneer soon after the adhesive has dried and before it has cured. if veneer is not applied until after the adhesive has polymerzed or cured, the bond may not be satisfactory.

After the veneer has been placed on the adhesive, the entire assembly, veneer and substrate is placed in a suitable hot-plate press. The pressure and temperature cycle of the veneer assembly in the press is important, as will appear hereinafter.

My process is directed particularly to the veneering ofy panels having a rotary-cut soft wood veneer face, such as Douglasfir plywood, in which there are striking differences in density in the component parts of the annual ring, that is, in the summer wood as compared to the spring Wood. I have found that the spring Wood, the low density portion of the annual ring is easily compressed. When thsrssvltant.Compressione. silbsequently. relieved, for @X1 ample by exposure to high humidity or moisture, the spring wood will raise relative to the summer wood, resulting in what is commonly known as grain raising. One of the `major liabilities of Douglas tir plywood is that it develops grain raising. In order to reduce grain raising,

it is imperative that compression of the panel during press- A ing in a hot-plate press be reduced to an absolute minimum.

Good bonding, however, requires firm pressure and superiorA bonding isv generally obtained with greater pressure. A balance must, therefore, be struck so that the pressure is'adequate to obtain good bonding but not greatv enough lto cause serious compression of the spring wood. In conventional plywood manufacture involvingy pressing time from'2 to 4 minutes, it is customary to use pressures from 150 to 250 p.s.i.

In making tests on the inter-relation between pressure, temperature and time, it was discovered that with an increase in time inthe press, andrat constant pressure, there is an increase in the compression of the panel. This is contrary to the commonly accepted principle that the panel v thickness `is not reduced in 2 to 4 minutes at pressures in the range of 150 to 200 p,s.i. In other words, increased time in the press results in increased compression of the spring wood which consequently leads to increased grain raising.

Table I below shows the effect of time on the amount of compression obtained when l/H inch birch veneer is bonded to a 57s inch Douglas r plywood at a constant pressure of 200 p.s.i., with dry urea adhesive as the bonding l medium. The amount of compression was measured for time intervals of 3, 6, 12 and 60 seconds. It will be noted from this table that the average compression of the panel when the birch veneer is applied to one side is about .003 inch for 3, 6 and l2 seconds. The compression is about .008 inch when the pressure is maintained for 60 seconds. When a pressure of 30() p.s.i. is used, the average compression for 3, 6 and l2 seconds at a temperature of 3209 F. is .005 inch. The compression is .010 inch Whenthe pressure is maintained for 60 seconds.

v Table l [Compression of -inch Douglas fir plywood subjected to hot-plate pressure in veneeringone side with l-134inch birch veneer, containing equilibrium moisture, placed across plywood veneer.

The bond obtained at pressures of 200 and 300 p.s.i.

was satisfactory on all the specimens pressed for at least 3 seconds. These tests indicate that a good bond can be obtained with,%4 inch veneer, pressed from 3 to 12 seconds when using a dry urea adhesive and a pressure of 200 p.s.i. and a temperature of about 320 F. Compressions in the range` of .008 inch to .010 inch are undesirable and, accordingly, time in the press must be kept within the limits specied in order to provide a llat panel. There was no measurable warping or cupping over a span of 6 inches in any of the panels tested.V

Table I also shows that a pressure of300 p.s.i. applied for no more than 12 secondsdoes not produce objectionable compression.

applied.

Table II below shows the compression and cupping ob- Press temperature It is well known that pressures of 30()- p.s.i.4 may not be usedvunder conventional Douglas iirv plywood manufacturing procedures in which .the time,Y under pressure is 2 to 4 minutes and the glue is f-reshly tainedwhen 1&4 -inc h'birch veneer is 'applied` to 1A inch`- ake board at 200 p.s.i. and 320 F. for 3, 6, 12 and 60 seconds. The increase in compression resulting from 60 seconds in the press as compared with 6 seconds is readily apparent. The compression was doubled. A further result of the test illustrated in Table Il is that the 1A: inch ake board does not stay as flat as Douglas r plywood when the latter is veneered across the grain.

Table Il [Compression of 1,d-iueh particle board subjected to hot-plate pressure in veneering one side with lei-inch birch veneer containing equilibrium moisture. Press temperature 320 F. Urea adhesive, dry. Time under pressure 3, 6, 12 and 60 seconds. Width of spccimen=6 inches] Results Pressure, p.s.i. Time Bond Comp, Warping,

inches inches 200 6 sec. (av.) Good 004 004 260 6() sec GoofL 008 009 Table III illustrates the effects when veneer is applied to Douglas tir plywood with the grain of the vencer parallel to the grain of the plywood faces. When the veneer is soV applied there is pronounced warping or clipping. The boards are not lat regardless ofthe time Yin the press, when the `veneer is applied with its grain parallel to the grain of the plywood faces, there is substantial warping.

Table III d [Warping of -inch Douglas fir plywood subiected to hot-plate pressure in veneering one side with Esi-inch birch veneer containing equilibrium moisture placed with the grain parallel to the grain of the faces of the plywood. Press temperature 320 F. Urea adhesive, dry. Time under pressure 3, 6, 12 and 60 seconds.. Width o specimcn=6 inches] Results.

Pressure, p.s.i. Time Bond Wai-ping,

inches 200 6 sec. (av.) Good--. .006 n 200 60 sec- Good--- 006 In addition to the warping problem, if thin veneers are I applied to Douglas ir plywood, they willnormally check in use. When the grain of the veneer is parallel to the grain of the Douglas fir veneer faces, the presence of the additional thin veneer face does not prevent checking. On

the other hand, thin veneers applied across the grain of i the Douglas ir veneer, prevents checking of the latter and the veener itself does not check. This is an important v result in that the checking of plywood and of the veneer faces applied thereto is a major problem in this industry.

In myrprocess I am able to greatly increase the capacity of the hot-plate press because my maximum pressing time is only l2ws`econds as compared to a minimum pressing time of `60seconds and a normal pressing time of 2 to 4 minutes for conventional processes. I also improve the panel from .the Apoint of view of grain raising since I `greatly reduce the compression resulting from hot-plate pressure. Furthermore, in the case of plywood, I can eliminate the checking of plywood which would check even with the veneer covering or facing if the veneer was not applied as disclosed herein.

A further important condition in using the process is the temperature of the interior of the plywood panel being veneered or the average temperature of the panel.

I have measured the temperature of Douglas r plywood A at various time intervals at a point approximately midway between the surface and thel center of the panel. This represents the approximate average temperature of the plywood at each time interval.v The relationship between the temperature at this point and the time under pressure with a platen temperature of 325 F. is shown in the attached drawing. Note that yfor a pressing time of l Y minute, the average temperature of the substrate is over 200 F. It is apparent from this curve that the average temperature of the substrate must not exceed about 200 F. if excessive compression is to be avoided. The surface temperature of the thin veneer quickly reaches the temperature of the platen but the temperature at the center of the substrate generally does not reach the temperature of the platen for several minutes. Permissible compression is obtained if the time under pressure is limited to that in which the average temperature of the panel is substantially below about 200 F.

A large number of tests were made in attempts to bond 1/24 inch birch veneer at 200 p.s.i. for 6 seconds and 60 seconds. The bond obtained in 6 seconds was not satisfactory, Whereas it was satisfactory at '60 seconds. Accordingly, it would appear that pressing times which result in good bonding with very thin veneer do not produce a satisfactory bond when veneers of conventional thickness are used. This result substantiates commercial experience in the use of veneers from 1/16` inch to V28 inch.

In summary, my process includes the step of forming a dry adhesive layer on one surface of a substrate which is to be veneered, whether it be plywood, hardboard, akeboard or the like. Preferably, the adhesive layer is formed by rst applying an aqueous solution of adhesive and then allowing the adhesive to dry, but not to cure. The veneer is positioned on the adhesive layer prior to curing of the adhesive and the entire assembly is then placed in a hot-plate press. When the panel or substrate to be veneered is plywood, it is preferred to place the grain of the veneer across the grain of the outer plies of the plywood.

The veneer and substrate assembly is maintained under hot-plate pressure for a period greaterthan 3 seconds, but not greater than 12 seconds, as described hereinbefore. The compression of the wood must be kept Within bounds. The temperature of the hot-plate press may be about 325 F., but as shown in FIGURE 3, the average temperature of the substrate should not be allowed to exceed about 200 F. In other words, as the average temperature of the substrate increases, it is an indication of excessive time in the press or increased compression. The pressure applied to the assembly may be in the range of 200 to 300 p.s.i., which is higher than many pressures commonly used in plywood manufacture.

Whereas the preferred form of the invention has been shown and described herein, it should be realized that there are many modilications, substitutions and alterations thereto within the scope of the following claims.

I claim:

l. A method of facing a board product composed of ligno-cellulosic bers with veneer which consists in forming a dry and uncured adhesive layer upon a surface of said board product, placing a veneer containing at least some of its equilibrium moisture and having a thickness of 1,48 inch to %,5 inch on the adhesive, then subjecting the assembly to hot-plate pressure to drive the water vapor into the adhesive, maintaining pressure, in the range of about 200 p.s.i. to about 30()u p.s.i., and heat upon the assembly for a period not less than three seconds nor more than 12 seconds and until the adhesive has been activated, but not long enough to raise the average temperature of the board product above about 200 F.

2. A method of facing a board product having a surface composed of ligno-cellulosic iibers with thin veneer which consists in applying an aqueous solution of an adhesive on the liber surface, drying the adhesive without curing it, placing a veneer containing at least some of its equilibrium moisture and having a thickness of S inch to 1,435 inch on the adhesive, then subjecting the assembly to hot-plate pressure, in the range of about 200 p.s.i. to about 300 p.s.i., to drive the water vapor into the adhesive, maintaining the hot-plate pressure for a period not less than three seconds nor more than 12 seconds and until the adhesive has been activated, but not long enough to raise the average temperature of the board product above about 200 F.

3. A method of facing a plywood panel having outer veneers that check when subjected to changes in moisture content which includes the steps of forming a dry and uncured adhesive layer on one surface of the plywood panel, placing a veneer containing hygroscopic moisture and having a thickness of 1/48 inch to 1&5 inch on the adhesive so that the grain of the veneer crosses the grain of the face veneer of the plywood panel, then subjecting the assembly to hot-plate pressure, in the range of about 200 p.s.i. to about 300 psi., to vactivate the adhesive by water vapor driven from the Veneer, maintaing said hot-plate pressure for a period not less than three seconds nor more than l2 seconds and until the adhesive has been activated, but not long enough to raise the average temperature of the panel above about 200 F.

4. The method of claim 3 further characterized in that said dry adhesive is formed by rst applying an aqueous solution of adhesive and then drying the adhesive without curing it.

References Cited in the tile of this patent UNITED STATES PATENTS 1,435,594 Elmendorf Nov. 14, 1922 1,894,218 Elmendorf Ian. 10, 1933 2,435,209 Elmendorf Feb. 3, 1948 2,724,675 Williams Nov. 22, 1955 2,827,935 Alexander Mar. 25, 1958 3,011,932 Downing Dec. 5, 1961 

1. A METHOD OF FACING A BOARD PRODUCT COMPOSED OF LIGNO-CELLULOSIO FIBERS WITH VENEER WHICH CONSISTS IN FORMING A DRY AND UNCURED ADHESIVE LAYER UPON A SURFACE OF SAID BOARD PRODUCT, PLACING A VENEER CONTAINING AT LEAST SOME OF ITS EQUILIBRIUM MOISTURE AND HAVING A THICKNESS OF 1/48 INCH TO 1/85 INCH ON THE ADHESIVE, THEN SUBJECTING THE ASSEMBLY TO HOT-PLATE PRESSURE TO DRIVE THE WATER VAPOR INTO THE ADHESIVE, MAINTAINING PRESSURE, IN THE RANGE OF ABOUT 200 P.S.I. TO ABOUT 300 P.S.I., AND HEAT UPON THE ASSEMBLY FOR A PERIOD NOT LESS THAN THREE SECONDS NOT MORE THAN 12 SECONDS AND UNTIL THE ADHESIVE HAS BEEN ACTIVATED, BUT NOT LONG ENOUGH TO RAISE THE AVERAGE TEMPERATURE OF THE BOARD PRODUCT ABOVE ABOUT 200*F. 